Quaternary coastal plains as reservoir analogs: Wave-dominated sand-body heterogeneity from outcrop and ground-penetrating radar, central Santos Basin, southeast Brazil

Abstract

Sub-seismic sand-body characterization depends largely on understanding the heterogeneities observed in outcrops of equivalent depositional settings to subsurface reservoirs (reservoir analogs). Outcrop information, however, rarely covers the whole depositional system. Ground-penetrating radar (GPR), on the other hand, is a powerful tool for the high-resolution visualization of sand-rich deposits in the shallow subsurface, allowing for the description of their internal architecture, relationships with depositional systems, and its larger scale stratigraphic evolution. The internal character of radar patterns can be interpreted with respect to facies seen in exposures, leading to the prediction of heterogeneities below radar resolution. In this paper we use GPR and outcrops to describe the heterogeneity of Quaternary coastal-plain deposits from central Santos Basin (Paraná state, southern Brazil) where Pleistocene and Holocene, wave-dominated regressive barriers and transgressive lagoons and estuaries were formed under the influence of Quaternary sea level and climatic changes. Sedimentary facies reveal the predominance of sand deposited under the influence of tidal processes, fair-weather and storm wave processes. Radarfacies indicate a tendency of progradation of beach-face reflectors over subtidal radarfacies within two regressive successions separated by a regional unconformity. Interpretation of GPR resulted in the definition of 13 radarfacies arranged in three associations reflecting major depositional systems: (1) strandplains formed by prograding beach/dune-ridge sets over subtidal deposits, (2) spit-inlets, with progradation of beach faces over subtidal deposits and longshore accretion of spits, and (3) paleolagoons/paleoestuaries. Sedimentary facies indicate that strandplains and spit-inlets are predominantly sandy, while a muddy composition is expected in paleolagoons/paleoestuaries. The distribution and evolution of these systems were controlled by base-level oscillations and/or sediment supply that resulted in the formation of two regressive barriers separated by a wave ravinement surface configuring two parasequences with a wide variety of inter- and intra-sand body heterogeneities. This arrangement imprints a relatively high degree of compartmentalization within deposits that are traditionally considered continuous and homogeneous. Therefore, the Quaternary coastal plain of Paraná can be used as an analog for compartmentalized coastal reservoirs.